1. Field of the Invention
The present invention is concerned with catalytic cracking of hydrocarbon streams in the absence of added hydrogen to produce gasoline and fuel oil. In particular, this invention is concerned with an improved process of the moving bed type, in which the improvement, in one aspect, resides in utilizing a platinum promoted cracking catalyst that converts coke to carbon dioxide with high thermal efficiency.
2. Description of Prior Art
Continuous catalytic cracking in moving bed systems is extensively used in petroleum refining to reduce the boiling range of relatively nonvolatile petroleum fractions, such as gas oil, thereby producing high quality gasoline and fuel oil. The process, as actually used, differs in particulars from refinery to refinery. However, all installations use 4 to 20 Tyler mesh size particles of a high surface area, inorganic porous acidic solid as catalyst; all operate in the absence of added hydrogen; and all utilize a continuous feed of the stream to be cracked, as well as continuous cyclical transport of an active inventory of catalyst between a regeneration zone and a cracking zone.
The cracking apparatus itself is sub-divided into a cracking section that includes a cracking zone, and a regenerator section that includes a regeneration zone or kiln. A suitably prepared hydrocarbon stream and hot regenerated catalyst are continuously passed through the reactor section under cracking conditions at elevated temperatures. During passage through the cracking zone, the hydrocarbon stream is cracked, and the active catalyst becomes deactivated by a deposit of substantially nonvolatile, combustible coke. The cracked products are separated from the deactivated, spent catalyst in some region in the cracking section, and are passed to a fractionation system for recovery of the desired fractions. Simultaneously, the spent catalyst is continuously passed to a regenerator section where it is contacted with air or with other oxygen containing gas at a temperature sufficiently high to burn the coke and thus regenerate active catalyst. During the combustion, the catalyst particles are reflexively heated, i.e., the particle acquires a major fraction of the heat generated by the combustion, so that the regenerated catalyst is considerably hotter than the spent catalyst. Within the regenerator section the regenerated catalyst is separated from the flue gas which is continuously discharged, and the hot regenerated catalyst itself is continuously discharged and returned to the reactor section where it provides not only the required catalytic activity, but also some or all of the heat required to vaporize and/or to heat the hydrocarbon feed to cracking temperature, as well as the heat that may be required to convert a feed to the cracked state. The cracking of a feed, in itself, is therefore endothermic because of these different heat requirements.
The process just described has evolved into a highly efficient operation with the advent of modern catalysts and improvements in the apparatus. Catalyst improvements, in particular, have made it possible to make more high quality gasoline at the expense of coke and gas, i.e., catalyst selectivity in the cracking section has been improved. However, there are persistent inefficiencies and problems associated with the regenerator section. These inefficiencies and problems come about because, in general, the flue gas formed in the regenerator contains large amounts of CO (carbon moxoxide) as well as CO.sub.2 (carbon dioxide).
It is known to those skilled in the art of catalytic cracking that only about four tenths of the total heat of combustion of a coke deposit is released in the kiln on conversion to CO. Thus, if the combustion in the regenerator produces one mole of CO for each mole of CO.sub.2, for example, only about seven tenths of the potential total heat of combustion is released; the remaining three tenths, or 30%, is carried by the latent heat of combustion of the CO, and may be released by the reaction of EQU CO+1/2O.sub.2 .fwdarw.CO.sub.2
In many installations the carbon monoxide in the flue gas is burned in a separate plant to make steam and/or reduce CO emissions to meet standards.
The incomplete combustion just described, of course, limits the reflexive heating of the catalyst and, in practical operation, limits the flexibility of the process and the overall efficiency achievable. For example, catalysts of very high selectivity (i.e. high efficiency) and/or low coke-producing feeds may not produce sufficient heat in the regeneration zone for optimal operation of the process. In one instance, for example, insufficient heat may be available to maintain an optimal cracking temperature in the cracking zone. In another instance, inefficient cracking may result from a temperature in the regeneration zone which is not sufficiently high to provide optimal regeneration (i.e., too high a residual carbon on generated catalyst). In still another instance, the temperature in the regenerator will become too low to support the combustion, and the burning stops altogether causing shutdown of the plant. In such situations, it would be highly desirable to increase the CO.sub.2 /CO mole ratio of the combustion products formed in the regeneration zone, thereby increasing the reflexive heating of the catalyst. While means other than reflexive heating may be employed, such as increasing the feed preheat for example, such means are more cumbersome and/or more costly or otherwise less desirable.
Additionally, when excess oxygen is contained in the combustion gases in the regenerator section, especially after separation of the catalyst from the flue gas, erratic ignition of the gaseous mixture (known as "afterburning") often can and does occur. This may lead to damage to the apparatus and/or catalyst from the resulting hot spots.
U.S. Pat. No. 2,436,927 issued Mar. 2, 1948, discusses the inclusion of a carbon monoxide oxidizing catalyst with the cracking catalyst in fluidized catalytic cracking process to eliminate afterburning. Copper, chromium, manganese, cobalt and nickel are recited therein as effective. It has been proposed to alleviate the CO problem in moving bed systems by adding a small amount of chromic oxide to the catalyst, as described in U.S. Pat. No. 2,647,860, issued Aug. 4, 1953. This is reported to cause a small but tolerable impairment in gasoline yield.